[Bug 199189] SWAP on ZFS can crash server

[bugzilla-noreply at freebsd.org]

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=199189

Chris M <cmangin at arobas.net> changed:

           What    |Removed                     |Added
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                 CC|                            |cmangin at arobas.net

--- Comment #17 from Chris M <cmangin at arobas.net> ---
I'd like to point out this page which has relevant information for those
wishing to tune the VM system: 

https://wiki.freebsd.org/AvgPageoutAlgorithm

The tunable vm.pageout_wakeup_thresh hasn't been mentioned here yet and seems
to be important for the way the the VM subsystem works.

So with that in mind, here's my current configuration for a 8Gb machine with
swap on zvol.

vm.v_free_severe=20460
vm.v_free_min=32768
vm.v_free_target=51200
vm.pageout_wakeup_thresh=36044
vfs.zfs.arc_free_target=36044

Note that I've picked:

vm.pageout_wakeup_thresh = vm.v_free_min * 11/10
vfs.zfs.arc_free_target = vm.pageout_wakeup_thresh

as these are the way they are calculated by default.

As a reference, the defaults values for my system are:

vm.v_free_reserved=2650 (untouched)
vm.v_free_severe=7642
vm.v_free_min=12634
vm.v_free_target=42586
vm.pageout_wakeup_thresh=13893
vfs.zfs.arc_free_target=13893

I submitted my system to memory pressure, notably by runs of poudriere
generating up to 3Gb of swap usage. I'm happy to report that my system remained
rock stable and very responsive during the tests.

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